1. Field
This patent specification generally describes an image forming apparatus. For example, it generally describes one capable of smooth transmission of recording medium.
2. Discussion of the Background
Background image forming apparatuses, such as printers, facsimiles, copiers, and multifunction apparatuses which print, fax, copy, and so on generally use an electrophotographic process for image forming. The electrophotographic process includes a charging process of an image carrier, a latent image forming process to form the image on the image carrier, a developing process in which toner is adhered, a transferring process which transfers the toner image to an intermediate transfer belt and transfers the toner image to the recording medium at a transfer device and a fixing process to fix the toner image at a fixing apparatus.
The transfer device forms a transfer nip by a pressuring force of a spring mechanism so as to contact a transfer roller with a pressure to the intermediate transfer belt which is the image carrier moving endlessly. The transfer roller is pressed in a direction of a thickness of the recording medium so as to give a pressure to any recording medium even if the recording medium is thick.
When the recording medium is conveyed to the transfer nip, a necessary space equal to a thickness of the recording medium is formed by a pushing force of a leading edge of the recording medium so that the recording medium pass through the transfer nip. If the recording medium is thin, the necessary space for the thickness of the recording medium is easily formed by a reform of a rubber layer of the transfer roller. The thin recording medium can pass through the transfer nip without stacking at the transfer nip.
If the recording medium is thick, it is not possible to obtain a necessary space for the thickness of the recording medium simply by the reform of the rubber layer of the transfer roller. It is needed that the transfer roller is retracted by the leading edge of the recording medium to form the necessary space. Otherwise, the recording medium can not enter the transfer nip and stack at a position immediately before the transfer nip until the necessary space for the thickness of the recording medium is formed.
If the stacking of the recording medium occurred for a relatively long time, a load to the intermediate transfer belt is increased due to a friction between the recording medium and the intermediate transfer belt. Due to the increase of the load of the intermediate transfer belt, a speed of the intermediate transfer belt may change. As a result, an uniformity of the color density which is called a shock jitter is occurred.